Modern twisted pair cable consists of two separate insulated wires paired through twisting means. Ideally, the insulation of each conductor is applied in such a manner that the concentricity of the conductor with respect to the insulation is maintained. When the conductors are paired, the electrical properties of the cable are maintained at a desirable level.
During installation and handling of the cables, the cables are bent, twisted and stretched, and this changes the geometry of the conductors with respect to each other and it adversely affects critical electrical properties such as attenuation, crosstalk and transmission. In recent years, with improved signal processing speeds and increased data rates, the need for high performance cables, up to 100 mbps, has greatly increased the use of twisted pair cables and has brought to light the importance of these inherent problems.
These installation and handling problems can be eliminated by joining the two conductors together by fusing the pair of conductors along its longitudinal axis, while maintaining each conductor's electrical properties, such that the geometry of the cable cannot change during installation or handling. This will maintain the electrical integrity of the cable.
Post extrusion chemical bonding is one method currently being used to make the cable. A chemical or solvent is applied to the surface of each individually insulated conductor such that, when they are brought into contact with each other and a catalyst is applied, the two separately insulated conductors are fused together. The product of this process is a cable of two insulated conductors joined along an axially extending groove and having desirable electrical properties. This approach offers limited processing speeds which are caused by an additional chemical or solvent application step, and a limited control of bonding characteristics because it is limited to the bond characteristics of the applied chemical or solvent. There also are limited material choices. Many thermoplastics have a very high resistance to some chemicals and solvents, which makes adhesion very difficult or impossible using these thermoplastics.
Another method currently used to negate the problem is parallel extrusion. In this process the two conductors are insulated by extruding the insulation onto the two conductors simultaneously, through the same head and tooling, while they are kept in close proximity to one another, such that the electrical properties of each individual conductor are maintained. The product of this process is a cable of two insulated conductors joined along an axially extending groove and having desirable electrical properties. This approach also offers limited processing speeds because of the nature of the extruder head tools used. It has a limited concentricity control because there is no way to adjust for normal tool wear and pressure variations in the extrudate. It has limited color coding capabilities because using one unmanifolded head limits the number of colors possible.